6b.5: Titration

While we are doing these titrations with base being added to acid, all could be done the other way around:

The strange jump in the middle is the result of pH being a logarithmic scale.

Each unit on the y-axis is a factor of ten, but not all factors of ten are the same.

For example, if we start with pH 2 and go to pH 3, it's a factor of 10: from 0.01 M to 0.001 M. We have to add 0.009 M worth of base.

To go from pH 5 to pH 6, it's also a factor of 10: from 0.00001 M to 0.0000001 M. We only have to add 0.0000009 M worth of base.

So what are the major features of note here?

1. The weak acid starts out higher than the strong acid. This makes sense, although bear in mind that this is only true if they're the same concentration2. That little bump at the start of the weak acid is a real feature.3. The equivalence point:

The pH is 7 for the strong acid, but basic for the weak acid. The pH of the equivalence point tells you whether your acid was strong or weak.

The volume is the same (once you account for some error in our data) for both. The AP test loves to ask about this, because you want to think it takes more for the strong acid, but that's not true: the equivalence point is the same volume for both (since their moles are identical).

We used 10.0 mL of 0.10 M acid. Because we used more than 10.0 mL of base to reach the equivalence point, the base must be less concentrated than the acid. If the base were more concentrated, we'd use less than 10.0 mL

3. Indicators. We use phenolphthalein to get the equivalence point, which doesn't actually change color at pH 7. That's ok, because the graph is so close to vertical--as long as it changes color at a pH where the graph is near-vertical, it will give the same volume.

They will almost certainly ask about what's actually in the solution, and you need to know in order to solve for pH:

Major differences:

Strong acid starts out completely dissociated. Weak is almost entirely together as a single molecule (this is definition of strong vs weak)

Why is the equivalence point for the weak acid basic? Because you made the conjugate of a weak acid...which is a base!